Wash Finish Concrete


Old 03-21-07, 01:23 AM
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Wash Finish Concrete

I am about to embark on the task of pouring a 17'x25' concrete pad next to my back patio for my utility trailer and boat. I have a friend that has all the trials and floats and I can form it up no problem. The question I have is my patio has a wash finish (not sure if this is the right terminology). When I added on to my back patio I contracted it out and they poured it then sprayed some chemical on it, and waiting for a period of time and washed it off with the water hose to expose the pea gravel on the surface. I am looking for some guidance on what chemical to use to achieve this result and how long I should leave the chemical on the surface of the concrete. I don't want to get holes and what not in my new pad. The main reason for this finish is to match my existing concrete it will butt up to. Any help will be much appreciated.

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Old 03-21-07, 04:27 AM
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The process is called exposed aggregate, or washed aggregate. It's pretty simple to do. First, if your existing concrete is a pea gravel mix, order the same for the new slab. If you are in a freeze climate, make certain it is air-entrained concrete!
Pour, screed, float, edge, and joint (groove) the concrete in the normal manner. As soon as the surface is smooth and free of defects, holes, etc., spray on a liquid surface set retarder with a pump-up garden sprayer. make sure you put it on heavy. These retarders can be bought from a local builder's supply. You will absolutely NOT find them at HD, Lowe's, Sherwin Williams, etc. One name brand is Euclid Surface Retarder "S". There are many others but this may be the most well known and readily available. These products drastically slow the set of the paste on the surface, but allow the concrete underneath to set as normal.
After the slab is sprayed, cover the whole thing with plastic sheeting. This insures against the liquid evaporating too quickly. When the concrete has reached a degree of set where you can walk on it without creating depressions, you can begin to wash the paste off. This is done with a garden hose and nozzle adjusted to a fan pattern. You don't want to blast the surface or you will pop the rocks out. At the same time you're washing, have someone use a soft push broom to help move the paste and sand off the slab. This is a very messy process. Avoid or re-direct runoff that will do damage or go into the storm sewers. It is illegal and environmentally harmful to wash concrete into storm drains.
Continue in this manner until the aggrgate is exposed across the whole slab. To make it look better, a few days later you can acid etch the slab with a solution of 10:1 water to muriatic acid and thorough scrubbing with the push broom and hose. This will remove the gray haze left over from the cement paste. A couple days later after the slab has thoroughly dried, seal the slab with an acrylic concrete sealer. Some "wet look sealers are made specifically to enhance the color of exposed agg. Some are even colored brown. Good luck.
Old 03-23-07, 10:56 PM
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Exposed aggregate - surface retarder

Been in the concrete finishing trade all my life (Dad was a finisher) and we have always used sugar water (1 lb./gal.) rather than the more expensive commercial stuff and covered it, as you said, with plastic, etc. Used it satisfactorily in all commercial as well as residential applications. Not to step on your toes, just a suggestion.
Old 03-24-07, 04:07 AM
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Yes you can do that, and I've done it too. You can also mix up molasses and water and use that. I thought about suggesting it but decided against it. You don't get nearly the same control as with the real (store bought) stuff. In addition, I've read technical reasons why you shouldn't use sugar in trade magazines, but I can't remember exactly what those reasons were. After I read the article, I just went back to using store bought stuff.
Old 03-24-07, 08:40 AM
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Wash Finish Concrete

Sugar is water soluable and the concetration can vary and it can possibly migrate down and weaken the concrete.

You will find many ready-mix trucks that always carry a bag of sugar in case of mixing drum problems.

Years ago, when in school. we would distract someone and dump sugar into their mix before they made cylnders for strength. It was great fun because sugar can really do a job on wet concrete.

Pecos - I agree that the chemicals made for exposed aggregate are more consistant and don't require the painful learning process for a DIYer. - One less thing to worry about on you first slab.

Old 03-24-07, 07:01 PM
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Not that you SHOULD, but you can also spray diesel to retard the surface and do an aggregate finish. It is better than sugar and worse than commercial retardants.

Be sure and tell the concrete company what you are doing so that they will use the proper aggregate, BTW.
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